[Util] Generalize external subtitle retrieval method(s).

Add void CUtil::GetPathsToLookForAssociatedItems()
Add void CUtil::ScanPathsForAssociatedItems(...)
Add int CUtil::ScanArchiveForAssociatedItems()
[Util] Make use of generalized ScanForExternalSubtitles(...) methods.
ScanForExternalSubtitles(...) now uses GetPathsToLookForAssociatedItems() and ScanPathsForAssociatedItems(...).
[Util] Remove obsolete method ScanArchiveForSubtitles() and sub_exts[].
[Util] Adapt ScanForExternalAudio(...) to use the generalized methods.
ScanForExternalAudio(...) now uses GetPathsToLookForAssociatedItems() and ScanPathsForAssociatedItems(...). ScanArchiveForAudio(...) is removed.